Looking for LED/Digital Readout Countdown Timer Circuit

[twchapman63]

Hi,

I am looking to build a LED/digital readout countdown timer that can interact with external projects and as I look through schematics I find it difficult to locate a good one. I thought where else better to ask than here? It would be much appreciated if someone could post a well done schematic below. Thanks!

 

[alec_t]

Probably cheaper, and certainly easier, just to buy one. They're widely available. Or is this for educational/fun purposes?

 

[twchapman63]

This is for educational purposes.

 

[ericgibbs]

This is for educational purposes.

hi,
Whats the max time range, accuracy and resolution thats required.?

 

[alec_t]

Well you hardly need a schematic. A micro, an LCD module and a couple of Rs and Cs and you're about there.
What timing accuracy do you need? How many display digits? What size display? What 'interaction' do you envisage?

 

[twchapman63]

Hi,

I am looking for accuracy to the second, two digit display (I am not sure about the display size) and to be able to trigger a transistor switch. Again I am a beginer so I really appreciate the help. A 10 second timer would be ideal. How difficult would it be to make one where the time delay is variable?

 

[alec_t]

That's certainly doable. Are you able to program a micro-controller to do the donkey work, or do you intend to use standard logic ICs?

 

[twchapman63]

I am able to do some programming, but I intend to use ICs.

 

[Llamarama]

Is it to count up or down?

If it's to count up I'd use a 4026, very simple to operate and standard CMOS.
If it's to count down I'd use a 4510 to make a BCD down counter, then a 4511 to convert BCD to 7 segment displays. This can also be used for an up counter if you should so wish.

The datasheets of these will show how to wire these up to accomplish this.

4026 https://www.ti.com/lit/ds/symlink/cd4026b.pdf
4510 https://www.doctronics.co.uk/pdf_files/HEF4510B.pdf
4511 https://www.doctronics.co.uk/pdf_files/HEF4511B.pdf

Whichever way you do it, good luck

 

[twchapman63]

Hi Llamarama,

I have two questions...do you know of any good circuit diagrams of something like this? and I have had a problem finding some other ICs for disributers, do you know any good distributers for ICs? Thanks a bunch!

 

[Brevor]

do you know any good distributers for ICs? Thanks a bunch!

For that it helps to know where you are located.

 

[alec_t]

Be aware that a single 4510 and 4511 will give a countdown from 9 (max), i.e. a single digit. Will that do? If not, you will need extra chips. You will also need a BCD rotary switch or some other switching arrangement if you want the count value to be adjustable.

 

[ericgibbs]

Hi Llamarama,

I have two questions...do you know of any good circuit diagrams of something like this? and I have had a problem finding some other ICs for disributers, do you know any good distributers for ICs? Thanks a bunch!

Hi,
This circuit could be easily modified for a two decade down counter.

Fit two , 4Bit switches to the parallel inputs of the LS192's and add a Load switch.

Link to a selection of counters.
https://www.electro-tech-online.com...asic-3-digit-led-decade-up-down-counter1.html

 

[Llamarama]

How do you want the circuit to count? Does it have to be exactly 1 second counts? Also, what country are you in?

I guess I'm lucky as I live about 5 miles from a good old fashioned electronics shop, but for online stuff I tend to use Rapid **broken link removed** or RS Components.

 

[twchapman63]

Im in the US. As far as the counts 1 second is prefered but it doesnt have to be super accurate. I am looking to have a two digit display. If I use 4510's or 4511's as the counters what should I use to interface them? Also, what should be used, an up or down counter if I want 10, 9, 8....going on?

 

[Llamarama]

Heres a 9 second count down that activates a relay once the count reaches 0, might be what you're looking for. http://www.bowdenshobbycircuits.info/page2.htm#cnt_down.gif

 

[alec_t]

Here's a possibility if you don't want to use a micro.
View attachment 66668

Circuit operation
Two BCD rotary switches are used to set the tens and units of the countdown value.
When the 'Go' button is pressed a 'go' signal (from the top end of R6) raises the PE pins of U2/U3 and this value is loaded into U2/U3 (CD4029s are used here instead of CD4510 because I don't have a simulation model for the latter, but the principle is the same although the pin-out will differ). A latch U1a is also set and the not-Q output enables down-counting by U2. On U2 counter roll-over U3 is decremented. When both counters are zero Q1 turns off and applies a 'clear' (aka reset) signal to latch U1a and a 'set' signal to latch U1b. The clearing of latch U1a stops further counting.
Latch U1b remains set until the 'Stop' button is pressed. Its complementary outputs can be used for driving additional electronic devices.

Edit: The circuit doesn't show them, but 100n decoupling caps should be connected across the power supply pins at each IC. Apologies for the haphazard component numbering. The 555 should have the identifier U8, not U1.